Current sources are known in the art. In theory, current source may be divided into several different categories. The most simple is a constant current source, which is configured to provide a fixed, time-invariant i Amperes of steady-state direct current (DC) regardless of input or output conditions. A variation is a fixed alternating current (AC) source, which provides a fixed, time-variant current for example of the form i(t)=i sin(t). Variable current sources are also known in the art, and may be usefully divided into current-controlled current sources, where i(t)=f (iref(t)); and voltage-controlled current sources, where i(t)=f(vref(t)).
Transconductors are also well known in the art. Transconductance is measured in Siemens, and is characterized by
      g    m    =            Δ      ⁢                          ⁢              I        out                    Δ      ⁢                          ⁢              V        in            for DC, and
      g    m    =            i      out              v      in      for small-signal AC.